1. Field of the Invention
The present invention relates to a photocatalyst composition. More particularly, the present invention is concerned with a photocatalyst composition which comprises (A) modified photocatalyst particles, the modified photocatalyst particles (A) being prepared by subjecting particles of a photocatalyst to a modification treatment with at least one modifier compound selected from the group consisting of different compounds each of which independently comprises at least one structural unit selected from the group consisting of a triorganosilane unit, a monooxydiorganosilane unit and a dioxyorganosilane unit; and (B) a binder component comprising a phenyl group-containing silicone, optionally containing an alkyl group. Photocatalysts, such as titanium oxide, are known to cause various substances to undergo decomposition by light energy and also known to increase the hydrophilicity of a surface by light energy. Therefore, the photocatalysts have been applied to the field of environmental clean-up, fields in which it is necessary to prevent dirt from adhering to the surfaces of various substrates, and fields in which it is necessary to prevent fogging from occurring on the surfaces of various substrates. When a film containing a modified photocatalyst is formed on the surface of a substrate by using the photocatalyst composition of the present invention, not only can the modified photocatalyst be strongly immobilized on the surface of the substrate without suffering a lowering of the activity of the modified photocatalyst, but also any of the film formed and the substrate covered with the film is not deteriorated by the action of the modified photocatalyst. Further, the above-mentioned film exhibits excellent properties with respect to durability, staining resistance, hardness and the like. Therefore, the above-mentioned photocatalyst composition is extremely useful for, e.g., preventing dirt from adhering to the surfaces of various substrates, and preventing fogging from occurring on the surfaces of various substrates.
The present invention is also concerned with a film formed using the above-mentioned photocatalyst composition, a functional composite comprising the film and a substrate covered with the film, and a shaped article produced by shaping the above-mentioned photocatalyst composition.
2. Prior Art
When specific types of substances are irradiated with light having energy which is larger than the energy gap (band gap) between the conduction band and the valence band of these substances, i.e., irradiated with light (excitation light) having a wavelength smaller than a wavelength corresponding to the band gap of these substances, the light energy causes an excitation (photoexcitation) of the electrons in the valence band, so that an electron and a hole are, respectively, generated in the conduction band and the valence band. By using the reducing activity of the electron generated in the conduction band and/or using the oxidizing activity of the hole generated in the valence band, various chemical reactions can be performed.
That is, when the above-mentioned substances are irradiated with excitation light, they can function as catalysts. Therefore, the above-mentioned substances are called “photocatalysts.” As a most representative example of such photocatalysts, titanium oxide is known.
As examples of chemical reactions catalyzed by the photocatalysts, there can be mentioned oxidative decomposition reactions of various organic substances. Therefore, when the photocatalysts are immobilized on the surfaces of various substrates, it is possible to cause various organic substances which have adhered to the surfaces of the substrates to undergo oxidative decomposition by light irradiation.
On the other hand, it is known that, when photocatalysts are irradiated with light, the hydrophilicity of the surfaces of the photocatalysts is increased. Therefore, when photocatalysts are immobilized on the surfaces of various substrates, it is possible to increase the hydrophilicity of the surfaces of the substrates by light irradiation.
In recent years, studies have been made for realizing application of the above-mentioned properties of photocatalysts to various fields, such as the field of environmental clean-up, fields in which it is necessary to prevent dirt from adhering to the surfaces of various substrates, and fields in which it is necessary to prevent fogging from occurring on the surfaces of various substrates. For enabling such applications, the method for immobilizing photocatalysts on the surfaces of various substrates has a great importance.
With respect to the method for immobilizing a photocatalyst on the surface of a substrate, various proposals have been made. For example, Unexamined Japanese Patent Application Laid-Open Specification No. Sho 60-044053 discloses a method for immobilizing a photocatalyst on the surface of a substrate, in which a thin film of a photocatalyst is formed on the surface of a substrate by sputtering.
Of these methods which have been proposed, one which has been attracting attention as being especially advantageous is a method for immobilizing a photocatalyst on the surface of a substrate, in which a composition containing a photocatalyst is coated on the surface of a substrate to thereby form a photocatalyst-containing film thereon.
In the above-mentioned method for immobilizing a photocatalyst by coating, it is required:
{circle around (1)} that the photocatalyst be strongly immobilized on the surface of the substrate without suffering a lowering of the activity of the photocatalyst, and
{circle around (2)} that any of the film formed and the substrate covered with the film formed be not deteriorated by the action of the photocatalyst.
For immobilizing a photocatalyst by coating, various methods have conventionally been proposed.
For example, Unexamined Japanese Patent Application Laid-Open Specification No. Sho 60-118236 discloses a method in which a sol containing a photocatalyst precursor (for example an organotitanate) is coated on the surface of a substrate, and the resultant film is calcined to effect gelation of the photocatalyst precursor, thereby converting the photocatalyst precursor into a photocatalyst while immobilizing the formed photocatalyst on the surface of the substrate. However, this method includes a step for forming crystal microparticles of a photocatalyst on the surface of the substrate, wherein this step requires calcination at high temperatures. Therefore, this method has a disadvantage in that, when the substrate has a large surface area, it is difficult to perform the immobilization of the photocatalyst.
As a method which uses a sol containing a photocatalyst (and which, hence, does not require a step for forming crystal microparticles of a photocatalyst), Unexamined Japanese Patent Application Laid-Open Specification No. Hei 6-278241 (corresponding to U.S. Pat. No. 5,595,813) discloses a method for immobilizing a photocatalyst on the surface of a substrate, in which a water-peptized sol of titanium oxide is coated on the surface of a substrate. However, since a titanium oxide sol cannot exhibit a film-forming property under moderate conditions, this method also requires calcination at high temperatures. Further, the film obtained by this method is brittle and easily broken and comes off from the substrate, so that it is impossible to allow the photocatalyst to exhibit its effect at the surface of the substrate.
Also, methods have been proposed in which a resin coating material containing a photocatalyst is coated on the surface of a substrate. For example, each of Unexamined Japanese Patent Application Laid-Open Specification Nos. Hei 7-171408 (corresponding to U.S. Pat. No. 5,547,823) and Hei 9-100437 discloses a method in which a photocalyst is incorporated into a resin coating material containing, as a film-forming element, a resin which is unlikely to be decomposed by the action of a photocatalyst, such as a fluoro resin or a silicone resin, and the resultant photocatalyst-containing coating material is coated on the surface of a substrate. However, in the resin coating materials used in these methods, the dispersion of the photocatalyst is poor and, therefore, the coating materials become white turbid. Further, for obtaining a good film by these methods, it is necessary for the coating materials to contain a large amount of a resin, and the use of a large amount of a resin poses a problem in that the photocatalyst is embedded in the film and, hence, cannot exhibit a satisfactory activity.
Further, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 9-314052 discloses a method which uses a combination of a resin coating material and photocatalyst particles having an increased wettability for the solvent contained in the resin coating material. In this method, the resin coating material is first coated on the surface of a substrate and, then, before the curing of the coating material is completed, the above-mentioned photocatalyst particles are applied onto the coating material. However, this method is disadvantageous in that not only is needed a cumbersome operation, but also a homogeneous and transparent film cannot be obtained. The above-mentioned patent document also discloses a method intended to simplify the operation, specifically a method in which a mixture of a resin coating material and photocatalyst particles having an increased wettability for the solvent contained in the resin coating material is coated on the surface of a substrate. However, the photocatalyst cannot be prevented from being embedded in the film simply by using photocatalyst particles having an increased wettability for the solvent, and most of the photocatalyst particles are completely embedded in the film and, hence, cannot exhibit a satisfactory activity.
Further, when an organic substrate, such as a plastic shaped article, a film or an organic coating, is used as a substrate for immobilizing a photocatalyst by the above-mentioned prior art methods, the obtained photocatalyst-containing film is accompanied by the following disadvantage. The photocatalyst-containing film formed on the organic substrate causes the oxidative decomposition of the organic substrate by the action of the photocatalyt and causes the interface between the organic substrate and the photocatalyst-containing film to be deteriorated; therefore, the photocatalyst-containing film is incapable of exhibiting high durability, i.e., cannot be maintained for a long time.
For overcoming the above-mentioned disadvantages of the prior art, the present inventors previously proposed a photocatalyst composition which comprises a modified photocatalyst obtained by modifying the surfaces of photocatalyst particles with a silicone having a small surface energy and also comprises a binder having a surface energy larger than that of the modified photocatalyst (see International Patent Application Publication No. WO 2000/30747 (corresponding to EP No. 1136125 A1)). When the photocatalyst composition is used to form a film on an organic substrate, the modified photocatalyst particles in the obtained film exhibit an anisotropic distribution in the thickness-wise direction of the film, namely the concentration of the modified photocatalyst particles is low around the interface between the film and the organic substrate and is high around the exposed surface of the film. Therefore, this photocatalyst composition forms a photocatalyst-containing film which exhibits high photocatalytic activity and which is free from the occurrence of the photocatalytic deterioration at the interface between the organic substrate and the photocatalyst-containing film. However, this photocatalyst composition has the following technical problem. Even when a silicone acrylic resin which is relatively insusceptible to photocatalytic decomposition is used as a binder having large surface energy, it is impossible to completely prevent the deterioration of the binder from occurring when the binder is exposed to the action of the photocatalyst particles for a long time. As a result, it is difficult to obtain an immobilized photocatalyst-carrying substrate having high durability, i.e., having the capability of being maintained for a long time.
Unexamined Japanese Patent Application Laid-Open Specification No. Hei 9-227831 discloses a photocatalyst coating composition which comprises a surface-treated photocatalyst and a siloxane as a film-forming element (i.e., a binder), wherein the surface-treated photocatalyst is obtained by coating photocatalyst particles with a hydrolysable group-containing silane derivative or a hydrolysable group-containing siloxane oligomer, to displace the isoelectric point of the surfaces of the photocatalyst particles towards the acidic range (thereby increasing the surface energy), to thereby impart excellent dispersion stability to the photocatalyst particles and thus enable the photocatalyst particles to be preserved stably for a long time in the siloxane used as a film-forming element (i.e., a binder). However, in a film formed from the above-mentioned photocatalyst coating composition, the photocatalyst particles do not exhibit self-stratifying or self-gradating property (i.e., the ability to exhibit an anisotropic distribution) and, therefore, the photocatalyst coating composition also has the above-mentioned problem that it is difficult to obtain an immobilized photocatalyst-carrying substrate having high durability.
Further, Unexamined Japanese Patent Application Laid-Open Specification No. 2001-64583 discloses a photocatalyst-containing coating composition for forming a stratifying or gradating film, the composition comprising photocatalyst particles, a silicone acrylic resin containing no phenyl group, and an aqueous solvent as essential components thereof. The stratifying or gradating film has a structure wherein an acrylic resin component, which is a hydrophobic component of the silicone acrylic resin, exhibits an orientation such that the acryl resin component is positioned between the hydrophilic photocatalyst particles (having large surface energy) and a substrate. Therefore, the acrylic resin (which is decomposable by the photocatalyst) is an essential component of the film and, hence, the durability of the film is extremely low.
Thus, no method has yet been known which is for immobilizing a photocatalyst on the surface of a substrate by coating and which satisfies both of the above-mentioned requirements {circle around (1)} and {circle around (2)} without a need of a cumbersome operation.
An object of the present invention is to provide a technique for immobilizing a photocatalyst, the technique satisfying both of the above-mentioned requirements {circle around (1)} and {circle around (2)} without a need of a cumbersome operation. Specifically, the object of the present invention is to provide a photocatalyst composition for producing a functional composite without a need of a cumbersome operation, wherein the functional composite is free from deterioration occurring at an interface between a substrate and a photocatalyst-containing film and from deterioration of the binder component in the photocatalyst-containing film, exhibits an excellent balance of hardness and flexibility (impact resistance), and has excellent durability such that the surface of the functional composite exhibits, by light irradiation, an increased wettability (i.e., increase in hydrophilicity and hydrophobicity) and/or the photocatalyst activity for a long time. Another object of the present invention is to provide a photocatalyst composition for producing a shaped article without a need of a cumbersome operation, wherein the shaped article exhibits, by light irradiation, an increased wettability (i.e., increase in hydrophilicity and hydrophobicity) and/or the photocatalyst activity for a long time.